1. Field of the Invention
This invention relates in general to an automatic gain control circuit and its related method. More specifically, the present invention relates to an automatic gain control (AGC) circuit for controlling a start-up time of an oscillator and a method thereof.
2. Description of Related Art
In the wireless communication field, since an on-off keying RF (radio frequency) receiver with a super-regenerative architecture has a better performance/cost ratio with respect to sensitivity and noise immunity, the on-off keying RF receiver is widely used, such as in remote-controlled model cars and in other remote-controlled electrical appliances, devices or equipments. In such circuit structure, for maintaining its performance under a best condition, it is necessary to use an automatic gain control circuit to control the start-up time of an oscillator.
FIG. 1 is a schematic diagram showing a signal modulation of a on-off keying RF transceiver. Referring to FIG. 1, when input data Din is “1”, its corresponding transmitted output signal Sout is a high level signal modulated by a carrier wave, while when input data Din is “0”, a level of its corresponding transmitted output signal Sout is 0.
FIG. 2 is a schematic block diagram showing a super-regenerative RF receiver, and FIG. 3 shows a signal relationship of the super-regenerative RF receiver. Referring to FIGS. 2 and 3, a saw-tooth wave generator 220 generates a saw-tooth wave output current Iq according to a quench control signal qnch. An oscillator 210 receives the saw-tooth wave output current Iq and an RF signal Srf to generate an oscillation output Sosc. When the received RF signal Srf is “1” that is a signal modulated by a carrier wave, the oscillation output Sosc of the oscillator 210 has an earlier oscillation time as shown in FIG. 3. When the received RF signal Srf is “0”, the oscillation output Sosc of the oscillator 210 has a later oscillation time as shown in FIG. 3. In this manner, received data is reproduced, so that subsequent circuits can restore received data.
In order to stabilize the oscillation output Sosc of the oscillator 210, the super-regenerative RF receiver further comprises an automatic gain control circuit 230 for controlling a magnitude of the saw-tooth wave output current Iq generated by the saw-tooth wave generator 220 and for adjusting the start-up time of the oscillation output Sosc of the oscillator 210.
One can refer to IEEE JSSC Vol. 36, pp. 440, “A low-power CMOS super-regenerative receiver at 1 GHz” for descriptions of a conventional automatic gain control circuit. In general, a gm-C low pass filter is used to extract an energy level of an oscillation envelope, so that a huge space is occupied. In addition, the automatic gain control circuit with the aforementioned structure in general requires a time of several bits to acquire a signal level, so that its response time is slower. Furthermore, the AGC circuit is always working on tracing signals, causing a large amount of noise.